Method and device for thermite welding at large water depths

ABSTRACT

Method and device to join the ends of two cylindrical objects by welding at deep water without the use of divers, where the ends of the two objects are connected with an inner casing inside an outer casing thereby creating a closed chamber around the joint between the inner and outer casings, evacuating the chamber and initiating a thermite welding process, the closed chamber thereby being filled with metal and establishing a mould sealingly connected with the outer surface of the two objects and abridging the distance between the two objects.

The present invention is related to a method and a device for underwaterwelding of joints with two bodies having substantially cylindrical crosssection, such as pipes.

By new installations and repairs of steel pipelines at large depths fromtime to time the need arises to connect the ends of two pipes. As oftoday the most used solutions for this operation is butt weldingperformed by divers working in a welding chamber or welding habitatwhich has to be established around the pipe joint. Today diving isallowed to 360 m in emergency situations, however not more than 180 m ispreferred. It is expected that limiting restrictions will be adapted todiving in the future.

Mechanical and hydraulic couplings also are used to seal pipe jointswithout welding. Such couplings may be installed without the use ofdivers, however huge installation frames are needed to adjust the pipesand move the couplings on the ends of the pipes.

Common for the two above mentioned methods, however, is that the ends ofthe pipes have to be very exactly adjusted to each other as to diameter,circularity, length and deviation from alignment.

With the method and the device according to the present invention, apossibility is provided for safe, secure and simple welding of pipe endsat large water depths without use of divers. With the present inventionthe tolerances for the adjustment of the ends of the pipes are increasedsubstantially as related to the above mentioned known methods. Even acertain angle in the pipeline may be accepted.

With the present invention, sealing of a pipe joint may be achieved in ashort time in relation to known methods and the invention is highlyadaptable and advantageous when laying new pipes as well as repairs ofexisting pipes under water.

The above mentioned restrictions and disadvantages connected with knownpipeline joint solutions are avoided with the method and the deviceaccording to the present invention, as defined by the features stated inthe patent claims.

The device according to the present invention comprises a compactwelding chamber covering the pipe joint and being adapted for thermitewelding by creation of a muff or ring sealingly connecting the twopipes.

Thermite welding as such is a well known technology used extensively forwelding of railroad rails and power lines. With the present invention,the thermite technology is used to weld pipe joints under water remotelycontrolled.

The device according to the present invention is specially designed tofulfil the requirements for the method and comprises a joint casing aswell as equipment being connected with the casing during the weldingoperation as well as the operations necessary before the welding itselfmay start. Several standard components are included in the hydraulicsystem which will not be described in detail, as will not be theequipment for injection of cement and possibly equipment adapted forreleasing of the equipment, thereby preparing some of the equipment forreuse. Furthermore standard equipment is assumed available comprising asupport vessel with crane capacities of at least 25 tons,heave-compensated. A remote operated vehicle is necessary for performingthe cutting work and observations by means of TV-cameras. Electric poweris supplied for heating and operation of hydraulic pumps as well as foroperation controls. Nitrogen is used in the sense of a protection gasmixture without oxygen to be adapted to the specific project dependingon the water pressure and other circumstances on site.

FIG. 1 discloses a longitudinal section through the two pipes 1 and 2which are to be connected by a seal proof joint, using the method andwelding device according to the present invention and FIGS. 2 and 3disclose enlarged the sealing ring in respectively an preoperational andan operative state.

The welding chamber consists of an outer connecting casing 8 and aninner connecting casing 5, with outer refractory lining 11 and innerrefractory lining 15 and sealing rings 16, 17, 18, 19, 20 and 21. Theinner casing 5 is permanently connected to the outer casing 8 by stiffconnecting bars not shown in FIG. 1. In order to get the inner casing 5into the outer casing 8, casing 8 is made of two parts which are boltedtogether and thereafter welded to be gas tight.

The present invention is adapted for use in connection with replacementof a short length of an existing pipeline. In such a case two jointsmade according to the present invention, are needed. In other caseslonger lengths of the pipeline is to be replaced or a new pipeline is tobe laid. Then it may be suitable to use the present invention for onlyone joint, whereas the other joint is connected on board a ship or inshallow water by other methods.

FIG. 1 discloses only one joint, such as a case where a damaged portionof a pipe is removed in a length of at least 12 m by means of a remoteoperated vehicle (ROV). The end of pipe 1 is lifted approximately 0.3 mand cut to be plane. The inner and outer edges are rounded to ensuresmooth and safe operation is of the equipment which is to be guidedthrough the pipeline in the future for maintenance reasons.

The concrete mantle 4 is removed approximately 0.6 m from the ends ofthe pipe. The circumference on the free ends are brushed with steelbrushes. The distance and the angle between the ends of the pipes are tobe measured.

On a support vessel a pipe section 2 with the correct length isprepared. The concrete mantle 22 on the section should be removedapproximately 3.5 m at each end. The circumference on the free ends aresand blasted approximately 0.6 m from each end. Toothed bars 3 arewelded in the longitudinal direction of the pipe approximately 0.7 mfrom each end. Welding chambers are arranged on each pipe end. Allequipment such as hydraulic pumps, cylinders, valves as well as pipes,hoses, sealings, water tank, nitrogen containers, concrete mixers and acrucible 14 should be secured in advance to the connecting casings 8 andtested functionally. Containers 6 are disclosed generally in FIG. 1. Thecasings 5 and 8 are to be tested by running them in both directions bymeans of hydraulic drive wheels 7 and the toothed bar 3. Thereafter thecasings 5 and 8 are placed in such a way that the ends of the pipe 2 arevisible outside the casing 8. A mixture of iron oxide and aluminumpowder, socalled thermite, is filled into the crucible 14. The mixturealso can contain certain additives to establish a steel qualitycorresponding with the steel to be welded. The crucible 14 is sealed andevacuated approximately to a vacuum.

The pipe 2 with the equipment is lowered to the seabed and arrangedbetween existing pipes 1. A clearance up to 100 mm may exist between thenew and existing pipe and it is an advantage that the pipes are arrangedin a slight curve in the horizontal plane before the joint is connected.This will allow for shrinkage due to the welding process and avoidsstress in the pipeline. Guiding wheels 9 and driving wheels 7 areactivated hydraulically and move the casings 8 on to the existingpipe 1. The casings 8 are centering the pipes by means of the guidingwheels 9. Circumferentially between the guiding wheels are arrangedhydraulically operated pins (not disclosed) maintaining the position ofthe pipes in the casing 8. With a ROV it has to be ensured that materialfrom the seabed has not entered the interspace between the pipe and thecasing 5 and 8, which possibly has to be flushed out.

Pretensioned rubber sealing rings 16, 17, 18, 19, 20 and 21 are filledwith special sand at vacuum pressures, see FIG. 2. Said rings areactivated by injecting special mortar in each ring. When the internalvacuum is replaced by an injection pressure which is somewhat higherthan the outer water pressure, the part of the sealing ring closest tothe pipe will be pressed out between the retainer rings 24. Thepretensioning will thereafter cause the part closest to the pipe to bedrawn towards the pipe in the same way as an elastic band, see FIG. 3.The internal volume in each ring is automatically supplied with specialmortar.

The volume to be filled with steel is now established, defined by thesealing rings 17 and 20 and the outer refractory lining 15 on the outercasing 8, the inner refractory lining 11 and the sealing rings 18 and 19on the inner casing 5, and the outer surface of the pipes 1 and 2. Thevolume is however still filled with salt water at the same pressure asthe surrounding water pressure. The salt water is evacuated by fillingthe volume with nitrogen at somewhat higher pressure than the externalwater pressure. This has to be made carefully, thereby avoiding leakagesthrough the sealing rings.

The volume thereafter is flushed with water from the fresh water tankand using nitrogen as a driving gas. The flush water should havesomewhat higher pressure than the outside water pressure to avoid riskthat salt water is leaking through the sealing rings.

The fresh water is evacuated from the volume by filling it with nitrogenwith a somewhat higher pressure than the outside water pressure. Heatingthreads 10 on the inner and outer refractory linings 11 and 15 areactivated and the contact surface of the pipes thereby being preheated.The injection mortar is hardened and the last remnants of water isevaporated. Nitrogen saturated with water is replaced by new and drynitrogen. The heating continuously has to be surveyed, avoiding that thepressure exceeds one atmosphere above the outside water pressure. Aspreheating is performed in an oxygen free atmosphere, a relatively hightemperature may be achieved without damaging the rubber in the sealingrings.

The thermite is ignited within the crucible 14 approximately at vacuum.Preheating of part of the thermite by means of a thermal element (notdisclosed) may be required before ignition. The heat development and thecreation of slag will lead to a certain increase in pressure. When thereaction is finished, the pressure within the crucible shall beequalized to the external water pressure by adding nitrogen or lettinggas out. The reaction in the crucible provides a steel temperature ofabove 2,400° C. The ideal moulding temperature is depending also on therelationship between the volume to be moulded and the contact area andshould be calculated for each single case by FE-analyzes. It is assumedthat in most cases it will be lower than 1,800° C. Therefore theoperation has to be delayed until the ideal temperature is achieved.Just before the temperature is correct, the pressure in the volume to bemoulded and also in the crucible 14, should be adjusted to be equal toor slightly higher than the external water pressure. Of this reasonthere exists a connection system comprising valves between the volume tobe moulded, the crucible 14 and the nitrogen containers.

The plug 13 is opened for the steel which is flowing down to the sealing12, melting the sealing and continuing through the connecting pipe 23 tothe volume to be moulded, thereby connecting to the outer surfaces ofpipes 1 and 2. The rubber walls in the sealings 17, 18, 19 and 20carbonize in the contact surface with the flowing steel, whereas theinternal, stabilized mass of sand and special mortar will create abarrier to the water until setting is completed. Carbonizing of therubber is assumed to give a minor unimportant pollution in the outersurface of the steel. After the steel has set it will still be coolingand contracting. The new welded ring will be shrinked to the pipe andlarge radial pressure stress will be created in the surface between thering and the pipe. This pressure withholds creation of cracks and ensurea gas tight connection.

The iron oxide inside the crucible 14 is replaced from 0 to 100% byother metal oxides or pure metals, thereby however maintaining areduction process similar to the thermite process.

Cooling of the steel also means that the pipe will shrink in thelongitudinal direction. If the pipes in advance create a shallow curve,no tensile stress will be created in the longitudinal direction. If thepipes are completely linear such tensile stresses may occur, dependingon the friction between the pipe and the seabed.

Within the outer sealing rings 16 and 21 there now will exist twovolumes with seawater permanently sealed and which therefore will not beof any danger as to corrosion. Further corrosion protection andsecondary sealing of the pipe joint can be achieved with injection ofcement mortar in volumes filled with seawater.

The casings as well as all equipment may be left on the seabed, it ishowever a possibility to take up to the surface the water tank, themixing battery, the pressure equalizing system, the crucible andhydraulic pumps if such parts are connected to one unit which can bereleased from the casings 8 and taken up to the surface.

Simplicity and reliability of the method makes it impossible to test theweld with conventional methods. It also is not possible to correctpossible errors. It is presumed therefore that the level of quality forthe device as well as for the method is established with a full scaleprototype and tests in advance. The cross section of the weld may beoverdimensioned in relation to the thickness of the pipe wall to ensurethat relatively large defects may have no importance. Additionally asecondary seal may be established by injecting mortar into the volumebetween the wall of the pipe and the casing 8.

Further FE-analysis may show that the inner refractory lining 11 may beomitted.

Two pipes having two alignment routes at an angle to s each other alsomay be welded together with the method according to the presentinvention. The angle between the pipelines may be in a vertical orhorizontal plane or in a combined plane.

Likewise may pipes be welded together having different outer diameters,the casing 8 having a corresponding shape. Furthermore the pipes neednot rest on a horizontal surface, e.g. the thermite welding process mayalso be utilized in cases where one or both pipes rests on an inclinedunderground, or if two pipes are standing free in the water.

The method according to the present invention is not limited to weldingof pipes, but can also be used for welding joints of solid cylindricalbodies.

What is claimed is:
 1. A method of joining the ends of two cylindricalobjects by welding at deep water without the use of divers, comprisingthe steps of connecting the ends of the two objects with an inner casinginside an outer casing thereby creating a closed chamber around thejoint between the inner and outer casings, evacuating the chamber andinitiating a thermite welding process comprising the reaction of ironoxide and aluminum powder, the closed chamber thereby being filled withmetal and establishing a mold sealingly connected with the outer surfaceof the two objects and abridging the distance between the two objects.2. The method according to claim 1 wherein said iron oxide is replacedby other metal oxides or pure metals to an extent of from 0 to 100%,thereby however maintaining a reduction process similar to the thermiteprocess.
 3. A device adapted to join the ends of two cylindrical objectsby welding at deep water comprising an inner casing, an outer casing,interior sealings in the outer casing, interior sealings in the innercasing, the sealings being adapted to clamp around the exterior of thetwo objects to be joined, the device further comprising a cruciblecontaining a mixture of iron oxide and aluminum powder which afteroperation fills the volume between the inner and outer casings and thesealings completely with metal.
 4. The device according to claim 3 wherethe outside of the inner casing and the inside of the outer casingcomprise refractory linings against the volume to be filled with steel.5. The device according to claim 3 wherein the crucible is closed by aplug and a seal, the seal withstanding the water pressure but meltingwhen the plug (13) is opened.
 6. The device according to claim 3 whereinthe sealings comprise rubber tori containing special refractory sand atvacuum and pretensioned to be held in place by retainer rings and to beclamped around the two objects by increasing the interior pressure andsupplying the increased volume by injecting sand and mortar from anexternal source.